Tube rolling mills

ABSTRACT

A rolling mill for seamless tubes, of the type having a central mandrel extending between the rolls of all the roll stands and which is surrounded, in use, by a tubular workpiece. The mandrel is held in a fixed position with respect to the roll stands and has an enlarged portion where it passes between the rolls of each roll stand. Successive enlarged portions are smaller in diameter so that a gradual reduction of the diameter of a workpiece is achieved as the workpiece passes through the rolling mill. The front end of each enlarged portion is curved so that in axial section the curve is substantially the same as the curve of the cooperating roll and the mandrel is so arranged that the two curves are substantially symmetrical about a line parallel to the axis of the mandrel and passing through the mid-point of the space between the mandrel and the roll so that both surfaces of the workpiece are substantially evenly drawn as the workpiece passes through each roll stand.

United States Patent [1 1 Montagna TUBE ROLLING MILLS [76] Inventor: Carlo Montagna, Via S. Martino 11,

Milan, Italy [22] Filed: June 3, 1971 211 Appl. Nola 1 19,421

[30] Foreign Application Priority Data June 25, 1970 Italy, ..69197 A/70 [52] US. Cl. ..72/209 [51] Int. Cl. ..B2lb 17/10 [58] Field of Search ..72/208, 209, 97

[56] Reierence's Cited UNITED STATES PATENTS 3,456,475 7/1969 Vom Dorp et al 72/209 3,577,756 5/1971 Cramer ..,.72/209 X 3,610,012 10/1971 Delans "72/209 1,092,848 2/1913 Schilling ..72/209 Primary ExaminerMilton S. Mehr Attamey-Sughrue, Rothwell, Mion, Zinn & Macpeak [5 7] ABSTRACT A rolling mill for seamless tubes, of the type having a central mandrel extending between the rolls of all the roll stands and which is surrounded, in use, by a tubular workpiece. The mandrel is held in a fixed position with respect to the roll stands and has an enlarged portion where it passes between the rolls of each roll stand. Successive enlarged portions are smaller in diameter so that a gradual reduction of the diameter of a workpiece is achieved as the workpiece passes through the rolling mill. The front end of each enlarged portion is curved so that in axial section the curve is substantially the same as the curve of the cooperating roll and the mandrel is so arranged that the two curves are substantially symmetrical about a line parallel to the axis of the mandrel andpassing through the mid-point of the space between the mandrel and the roll so that both surfaces of the workpiece are substantially evenly drawn as the workpiece passes through each roll stand.

1 Claim, 4 Drawing Figures PATENIED HAYES I975 SHEET 1 [IF 3 INVENTOR Q A 5 GMAMA m KW 0K2 flu n BACKGROUND OF THE INVENTION The present invention relates to rolling mills used for rolling seamless metal tubes.

In particular this invention relates to a continuous rolling mill for the manufacture of seamless metal tubes, of the type having a number of rolling stands, each provided with two counter-rotating rolls, and a mandrel extending between each pair of rolls, over which is rolled a tubular workpiece from which the seamless metal tube is to be made.

In such continuous rolling mills of a known type, the workpieces are passed through successive pairs of parallel rolls, each of which pairs is placed perpendicularly to the adjacent pairs. The rolls of each pair rotate in opposite directions and have, in axial section, a concave curve so that each roll contacts the workpiece over a substantial arc; the outer diameter of the tube being determined by the curve of the'rolls and their separation.

The inner diameter of the tube is, on the other hand, determined by a mandrel which is introduced into the inside of the tubular workpiece. The mandrel moves along with the workpiece and, at the end of the rolling process, it is extracted from the finished tube by means of an extracting device.

During rolling a number of mandrels are used in turn; they all have a considerable length and a constant diameter. Continuous rolling mills of the type described above have a number of disadvantages.

First of all, the normal mandrels present problems both of bulk and cost since it is essential to have an extracting device designed to extract them from the finished tube. Moreover maintenance of the mandrels is particularly troublesome because, once worn, they have to be completely re-tumed and their diameter being decreased, they soon become unusable.

Finally, cylindrical mandrels with constant diameter bring about a poorly balanced reduction in the thick ness of a workpiece during rolling.

SUMMARY OF THE INVENTION According to the present invention there is provided a continuous rolling mill for the manufacture of seamless metal tubes, of the type having a plurality of roll stands, each provided with pair of counter-rotating rolls, and a mandrel extending between each pair of rolls, the mill being arranged so that when in use a workpiece surrounds the mandrel and extends between each pair of rolls, characterized in that the mandrel is axially fixed in relation to the roll stands and consists of a rod having a plurality of enlarged sections the end potion of each enlarged section which, in use of the mill, first comes into contact with the workpiece having an outwardly convex ogive shape in axial section.

Preferably the mandrel is so shaped, and in operation of the mill, is so located with respect to the cooperating roll cylinders that'in axial cross section the shape of that part of the curved portion of the mandrel which is in contact with the workpiece is substantially the same as the axial cross sectional curve of the cooperating part of the surface of the roll which, at any given time, is in contact with the workpiece, and the two said curves are substantially symmetrical about a line parallel to the axis of the mandrel passing through the midpoint of the space between the roll cylinder and the enlarged portion of the mandrel.

Other features and advantages of the present invention will become apparent from a consideration of the following description, which reference to the accompanying drawings which is presented purely by way of non restrictive example.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic perspective view from above and one side, of a continuous rolling mill constructed according to this invention;

FIG. 2 is a diagrammatic longitudinal cross section passing through the axis of the spindle of the rolling mill illustrated in FIG. 1, in which, for the sake of clarity, adjacent pairs of rolls have been shown with their axes parallel instead of perpendicular;

FIG. 3 is an enlarged detail of FIG. 2;

and FIG. 4 is a detail similar to that illustrated in FIG. 3 showing a corresponding part of a known continuous rolling mill.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIGS. 1 and 2, a continuous rolling mill, generally indicated 1, and comprising a number of rolling stands 2, aligned with a mandrel 3 passing through the rolling stands 2 is shown, The rolling stands 2 each consist of a frame 4 carrying two substantially cylindrical rolls 5, parallel to each other, and arranged for relative counter rotation with their axes perpendicular to the axis of the mandrel 3. The mandrel 3 passes through the space between the rolls 5 of each stand 2.

The space between the rolls of each pair decreases with the distance of the stands from the front part of the rolling mill 1.

In order to obtain a uniform milling, the axes of the rolls 5 of adjacent stands 2 are substantially perpendicular.

The mandrel 3 consists of a cylindrical portion 6 extending forwards from the first roll stand 2 and clamped at its front end, to the inside by a vice 7. The vice 7 holds the mandrel 3 rigidly in place with respect to the stands 2 while the substantially tubular workpiece 8 is being worked by the rolls 5.

As will be seen from FIG. 2 the mandrel 3 has a number of enlarged sections 9 joined by sections of reduced diameter 6. Each enlarged section 9 has a coaxial cylindrical center portion 10, and a front portion 11, which in axial section is outwardly convex and of ogival form, joining the enlarged center portion 10 to the surface of the portion 6. Each enlarged section is positioned in the space between a pair of rolls 5, and the front ogival portion 11 of each enlarged section 9 is so located with respect to the associated pair of rolls 5, that during working, the surface of the mandrel and the surface of the rolls 5 cooperate. In axial section the ogival curve of the mandrel 3 is substantially symmetrical with that part of the surface of the roller which is in contact with the opposite surface of the workpiece about a line parallel with the axis of the mandrel and passing through the mid-point of the space between the mandrel 3 and the roll 5.

At the commencement of the rolling process, the vice 7 is opened so that it is possible to draw the workpiece 8, on to the portion 6 of the mandrel 3 which extends out from the stands 2. The vice is then tightened again to lock the mandrel 3 axially with respect to the rolling stands 2, and the tubular workpiece 8 is then forced over the cylindrical portion 6 until its end comes into contact with the rolls of the first stand 2. The workpiece 8 is nipped between the rolls 5 and the mandrel 3 and urged forward over the mandrel 3 by the counter rotating rolls 5.

The differences and advantages of the continuous rolling mill 1 as against a known continuous rolling mill, will be described in relation to FIGS. 3 and 4 which show part of a rolling stand 2. Between the rolls 5 (only one shown) of the stand 2, illustrated in FIG. 4 there extends a mandrel 12 of the known constant diameter type.

With reference to FIGS. 3 and 4 it will be seen that the zone of contact between the tubular workpiece 8 and the rolls 5, called the zone of grip, extends over an arc of the surface of the rolls 5 which subtends an angle A at the center of the roll. This angle can be termed the angle of grip".

As can be seen from FIGS. 3 and 4 the angle of grip A is substantially identical in both cases. However, the angle of grip A can be subdivided into two angles, marked B and C respectively. The angle B represents an arc on the surface of the rolls 5 over which the tubular workpiece 8 undergoes deflection so as to form a portion 13 of the workpiece which is substantially frusto-conical. In the case of the known type of rolling mill the reduction in the outer diameter of the tubular workpiece 8 is accompanied by an increase in the thickness.

The angle C, on the other hand, represents an arc on the surface of the rolls 5 over which the thickness of the workpiece undergoes a substantial reduction.

When a mandrel of the known type is used, as is the case in FIG. 4, the angle B is relatively small with respect to the angle C: in other words, the proportion of the length of the frusto-conical portion 13 in which the workpiece wall is deflected so that the diameter of the workpiece as a whole is reduced is relatively small with respect to the proportion in which the wall thickness of the workpiece is reduced. With reference to FIG. 3 it will be seen that with a rolling mill constructed in accordance with the present invention the angle A is approximately the same as the angle B that is, workpiece diameter reduction and wall thickness reduction occupy approximately the same length of the workpiece.

Thus it will be appreciated that in the rolling mill 1 which is the subject of this invention the material of the workpiece 8 is more evenly worked at the zone of contact between the workpiece 8 and the rolls 5.

Since the frusto-conical section 13 is more extended in the case of the rolling mill 1 which is the subject of this invention, the reduction of the wall thickness of the workpiece 8 is then more even, and there are fewer variations in the wall thickness than was possible using known rolling mills. The reason for this will be more readily appreciated from a consideration of FIG. 4, from which it will be seen that the reduction in the wall thickness of the workpiece 8 which is achieved over the angle C occurs wholly by the action of the roll 5 on the outer surface of the workpiece 8. In the rolling mill 1,

on the other hand, both the outer surface of the cylinders 5 and also the outer surface of the ogival portion 1 1 of each enlarged section 9 of the mandrel 3 contribute to the reduction of the wall thickness of the workpiece 8. In fact these two surfaces substantially constitute a screw-on cone for the workpiece 8 so that the reduction in wall thickness occurs both at the outer surface of the workpiece 8 and also at the inner surface of the workpiece 8.

The outer and inner reductions in thickness, which are achieved in the rolling mill 1 are marked E and F respectively in FIG. 3; they are substantially equal and their sum is equal to the reduction D which is obtained on only one side of the workpiece in a rolling mill of the conventional type.

In the case of the rolling mill 1, which is the subject of the present invention, the reduction in thickness is evenly balanced on each side of the workpiece wall and, since it is effected both on the outer and on the inner surface of the workpiece 8, the material of the workpiece is evenly drawn thus eliminating the disadvantages due to excessive localized compacting of the material which occurred with previously known rolling mills.

On the other hand, the maintenance of the mandrel 3, is simple and inexpensive because the only portions which get worn are in effect the ogive portions 11. These can very easily be re-shaped, such as by turning without reducing the diameter of the cylindrical portion 10 of the enlarged section 9.

Naturally, the principle of the invention remaining the same, the details of construction my be widely varied in regard to what has been illustrated and described purely by way of non-restrictive example, without nevertheless going beyond the scope of this invention.

What is claimed is:

1. A continuous rolling mill for the manufacture of seamless metal tubes comprising a plurality of roll stands, a pair of counter-rotating rolls on each of said roll stands, a mandrel extending between each said pair of rolls and being axially fixed relative to said roll stands for receiving a workpiece which surrounds said mandrel for movement relative to said mandrel between each said pair of rolls, said mandrel comprising a rod having a plurality of enlarged sections, the end portion of each of said enlarged sections facing said oncoming workpiece having an outwardly convex ogive shape in axial section, each of said enlarged sections of said mandrel being so shaped and located with respect to said cooperating rolls that in axial cross-sections, the shape of said outwardly convex ogive curved portion of each enlarged section of said mandrel facing said oncoming workpiece is substantially the same shape as the curve of the cooperating part of the surface of said roll which at any given time is in contact with said workpiece and said two curves are substantially symmetrical about a line parallel to the axis of said mandrel passing through the midpoint of the space between said roll and said enlarged portion of said mandrel.

t II! t l 

1. A continuous rolling mill for the manufacture of seamless metal tubes comprising a plurality of roll stands, a pair of counter-rotating rolls on each of said roll stands, a mandrel extending between each said pair of rolls and being axially fixed relative to said roll stands for receiving a workpiece which surrounds said mandrel for movement relative to said mandrel between each said pair of rolls, said mandrel comprising a rod having a plurality of enlarged sections, the end portion of each of said enlarged sections facing said oncoming workpiece having an outwardly convex ogive shape in axial section, each of said enlarged sections of said mandrel being so shaped and located with respect to said cooperating rolls that in axial crosssections, the shape of said outwardly convex ogive curved portion of each enlarged section of said mandrel facing said oncoming workpiece is substantially the same shape as the curve of the cooperating part of the surface of said roll which at any given time is in contact with said workpiece and said two curves are substantially symmetrical about a line parallel to the axis of said mandrel passing through the midpoint of the space between said roll and said enlarged portion of said mandrel. 